Disclaimer: I work on a multisig wallet/service.

The question of why multisig is better becomes clear IMO when we approach self custody from a comprehensive risk analysis perspective.

What is the most robust way to hold bitcoins to ensure that you can secure them not just for yourself, but also future generations? What is a way to eliminate as many single points of failure (SPOFs) as possible and to be extremely forgiving to mistakes, mental or otherwise?

We’re still in the very early phase of Bitcoin. As adoption grows, attacks on Bitcoiners will undoubtedly get more sophisticated. From phishing attempts, supply chain attacks, widespread surveillance cameras and IoT devices, to social engineering. Hence it’s worth thinking through the risks that might exist not just today, but in the near future.

Multisig also enables innovative usages such as collaborative custody, which others have mentioned here in this thread. If we think about it, Bitcoin is the first and only asset in history that can be jointly owned by multiple parties without relying on a central authority. To me this is a big deal, and a super underrated property of Bitcoin that I think people will only come to fully appreciate in the years to come. Both in terms of security and more innovative use cases. Non-KYC estate planning is another.

And I don’t believe either singlesig or singlesig + passphrase is a good enough substitute for multisig w.r.t. safe custody. Wrote more about it here: https://hugonguyen.medium.com/passphrases-are-not-the-solution-for-bitcoin-custody-4f967a339b1b